The present invention relates to a method for producing ceramic green sheets, and particularly to a method for producing ceramic green sheets suitable for the production of precision ceramic laminate substrates, and a method for producing ceramic laminate substrates of high precision which comprises laminating the ceramic green sheets produced by the above method of production of ceramic green sheets, and firing the laminate.
As a method for controlling firing shrinkage of ceramic molded products, the inventors proposed in U.S. Pat. No. 5,753,160 issued on May 19, 1998 a method for controlling the firing shrinkage of ceramic molded bodies which comprises subjecting a ceramic powder to a pre-treatment to obtain a ceramic powder having a spherical diameter Rs of 1 xcexcm or less represented by Rs (xcexcm)=6/xcfx81S (wherein xcfx81 denotes a true density (g/cm3) of the ceramic powder, and S denotes a BET specific surface area (m2/g)), then subjecting the resulting ceramic powder to a heat treatment at a treating temperature previously calculated, and thereafter molding and firing the ceramic powder.
Furthermore, as related art, the inventors proposed in U.S. Ser. No. 09/080,293 filed May 18, 1998, now U.S. Pat. No. 6,051,171, which is a continuation-in-part of the U.S. patent referred to above a method of producing a fired molded body which comprises the steps of:
(a) molding a ceramic powder (A) having a spherical diameter Rs of 1 xcexcm or less expressed by the formula Rs (xcexcm)=6/xcfx81S (wherein xcfx81 denotes a true density (g/cm3) of the ceramic powder, and S denotes a BET specific surface area (m2/g)) and having an average degree of aggregation (X) of 2 to 10 expressed by the formula X=Rm/Rs (wherein X denotes an average degree of aggregation before heat treatment, Rm denotes a median diameter (xcexcm) before heat treatment measured by a laser scattering method and Rs denotes a spherical diameter (xcexcm) of the ceramic powder), and
(b) firing the green molded body at a predetermined firing temperature;
said method being characterized in that the rate of firing shrinkage of said green molded body during the firing step (b) is controlled to a desired value A1 by heat-treating ceramic powders at a temperature T1that can provide said firing shrinkage value A1, said temperature T1 being determined based on an established correlation between an amount of firing shrinkage at said predetermined firing temperature and a heat-treating temperature,
said correlation being obtained by the following steps:
(i) subjecting a ceramic powder (B) having Rs of 1 xcexcm or less and an average degree of aggregation of 2-10 to heat treatments before molding at at least two temperatures, said ceramic powder (B) having a composition similar to said ceramic powder (A) (in such a manner that a total amount of the greatest common contents of individual components common between said both powders) is 90% by weight or more, and satisfying such a relation that Rs of said ceramic powder (A) is within the range of xc2x130% of Rs of said ceramic powder (B) and the average degree of aggregation of said ceramic powder (A) is within the range of from xc2xd to 2 fold relative to that of the ceramic powder (B), then producing a molded body, and firing the molded body at the predetermined firing temperature to obtain firing shrinkage values corresponding to said at least two heat treating temperatures, thereby obtaining a regression line for the correlation between the heat-treating temperature and the firing shrinkage,
(ii) subjecting a small amount of the powder (A) to be molded in the step (a) to one heat treatment before molding, molding the powder, and firing the molded body at the predetermined firing temperature to obtain a firing shrinkage value at said one heat treating temperature, and
(iii) on the basis of the firing shrinkage value of the powder (A) obtained in the (ii) and the regression line obtained in the (i) on the powder (B), establishing said correlation by assuming that its regression line for the relationship between the heat-treating temperature and the firing shrinkage value of the powder (A) is correspondingly offset, in the direction of the axis for the firing shrinkage value in a graph obtained on the ceramic powder (B).
The above two methods have clarified a method of controlling the firing shrinkage of green sheets, in other words, can be said to propose methods for producing green sheets which can be controlled in firing shrinkage. However, when the methods are applied as they are, there are problems that green sheets diminished in occurrence of cracks and superior in lamination property cannot be obtained. On the other hand, for producing ceramic laminated substrates which are high in strength and superior in surface roughness even though being multilayered and precise, it is necessary to use a ceramic powder fine in particle size. However, when ceramic green sheets are produced using the powder of fine particle size, there are problems that cracks occur and ceramic laminated substrates cannot be produced, or green density of the green sheets (density of green sheet) markedly lowers and the laminated substrates are apt to be distorted at the time of lamination. Furthermore, the above problems per se can be solved by using raw materials subjected to dry grinding, but green density of ceramic green sheets becomes too high and adhesion between layers in lamination is inferior.
An object of the present invention is to provide a method for producing ceramic green sheets for making a ceramic laminated substrate which is reduced in occurrence of cracks in the green sheets and improved in adhesion between laminated layers after firing of the substrate, high in strength, and excellent in surface roughness, by use of a dry ground ceramic powder having a fine particle size with controlling the void content and content of organic components in the ceramic green sheets within desired ranges. Another object of the present invention is to provide a method for producing a ceramic laminated substrate excellent in adhesion between laminated layers, high in strength, and excellent in surface roughness by laminating and firing the ceramic green sheets obtained above.
As a result of intensive research conducted by the inventors in an attempt to attain the above objects, it has been found that the above objects can be attained by preparing ceramic green sheets having a predetermined void content and a predetermined organic component ratio by a wet method using a ceramic powder adjusted to have a spherical diameter within a predetermined range by dry grinding and subjected to a heat treatment at a predetermined temperature to adjust the average degree of aggregation Xxe2x80x2 to a predetermined range. Thus, the present invention has been accomplished.
The method for producing a ceramic green sheet according to the present invention is characterized by comprising:
a step of subjecting a ceramic powder to dry grinding so that a spherical diameter Rs satisfies 0.01xe2x89xa6Rsxe2x89xa60.5 [wherein Rs (xcexcm) means 6/xcfx81cS (in which xcfx81c denotes a true density (g/cm3) of the ceramic powder, and S denotes a BET specific surface area (m2/g) of the ceramic powder)],
a step of heat-treating the dry ground powder at a temperature of 250-1000xc2x0 C. to adjust an average degree of aggregation Xxe2x80x2 represented by the formula Xxe2x80x2=Rxe2x80x2m/Rs [wherein Rxe2x80x2m means a median diameter (xcexcm) of the ceramic powder measured by a laser scattering method after the heat treatment and Rs (xcexcm) has the same meaning as defined above] to a range of 2.5-15,
a step of mixing the thus heat-treated powder with organic components in such an amount that the organic components occupy 20-55% of the ceramic green sheet in volumetric ratio and the void content of the ceramic green sheet satisfies 5-25% and a solvent in a necessary amount, thereby preparing a ceramic slurry, and
a step of molding the ceramic slurry into a ceramic green sheet.
Furthermore, the present invention provides a method for producing a ceramic laminated substrate, characterized by laminating the resulting ceramic green sheets and firing the laminate.